There exist data processing or computer systems operating under a backup control scheme wherein a backup processor takes over from the operating processor if the latter develops a failure. For example, in Japanese Patent Laid-Open No. 75843/1988, there is a backup switching scheme involving a backup CPU for detecting a failure of an operating CPU that sends a break signal to the operating CPU (e.g., "000 . . . 0") via a serial interface. In turn, the operating CPU performs a suitable function to measure the bit length of the break signal, and then attempts to recover from the failure by issuing an interrupt signal and a reset signal. If the attempt to remedy the failure is unsuccessful, the failed CPU disconnects from the system.